In conventional on-glass voltage fed vehicle antennas such as disclosed in Parfitt U.S. Pat. No. 4,238,799, an adjustable LC network is required at the vehicle window. Adjustability of the components is necessary because of the tolerances with respect to the inductor and capacitor of the network. Typically, variable capacitors are used so that the network can be adjusted for resonance. It is necessary that both the network and the antenna be in tune. However, when the network is adjusted, the length of the antenna may also have to be adjusted. Often the antenna will be adjusted to an erroneous length which will require compensation of the LC network to bring the assembly to resonance, indicating to the operator that the tuning is proper, but there is a loss of gain resulting from the compensating action. In other words, because of the two variables, compensation of one of the variables may result in less gain than optimum although to the operator there may appear to be a proper match.
In Blaese U.S. Pat. No. 4,658,259, a current fed antenna is disclosed which obviates the requirement for the LC network at the window. This current fed antenna is very well suited for use with relatively high frequencies, such as used with cellular telephones. However, with lower frequency antennas such as in the citizens band (27 megahertz) the current fed antenna disclosed in Blaese U.S. Pat. No. 4,658,259 is impractical. This is because the lower frequency antenna requires an extremely large ground plane, of a magnitude that may be so large as to cover the entire back window of the vehicle.
It is an object of the present invention to provide an on-glass antenna which is useful with lower frequencies as well as higher frequencies and which does not require a matching LC network at a coupling box that connects the antenna to the window.
Another object of the invention is to provide an on-glass antenna for a vehicle which does not require adjustment of any LC network, thereby providing only a single variable for adjustment, i.e., the length of the antenna.
A further object of the present invention is to provide an on-glass antenna for a vehicle that is simple in construction and easy to manufacture.
Other objects and advantages of the present invention will become apparent as the description proceeds.